This invention relates to methods of coating materials intended for implantation and to the coated materials. More particularly, it relates to the deposition of fluoropolymer coatings on the surfaces of tubular and other substrates to improve the substrate's biocompatibility properties.
Devices intended for use within the body such as vascular prostheses often comprise materials which have limited biocompatibility and non-thrombogenic properties. The surfaces of such devices may be coated to improve biocompatibility without compromising other properties such as durability and flexibility. One technique for coating such materials is plasma polymerization. The surfaces of tubes comprising polymeric materials can be modified by deposition of a thin layer of a fluorine-containing polymer using plasma polymerization techniques, also known as glow discharge. The technique involves introducing a polymerizable organic monomer in the gaseous state into a vacuum containing the substrate material to be coated. The gas is then subjected to an electric discharge to initiate polymerization reactions, generating ions or free radicals which react with and deposit on the substrate. The polymer formed is normally deposited as a thin layer over the substrate material present in the reaction vessel. The bulk substrate characteristics are preserved, but the surface properties, which are major determinants of biocompatibility and non-thrombogenicity, can be modified or improved by plasma polymerization.
Plasma polymerization was used to prepare biomaterials as early as 1969 (J. R. Hollahan, et al., J. Appl. Polymer Science 13, 807, 1969). A leader in the area, Dr. H. Yasuda, conducted blood coagulation studies on glow discharge (plasma) polymer surfaces in 1976. A. W. Haln investigated biomedical applications as reported in an NBS Spec. Publ. 415 (May 1975). More recently, other investigators have also reported the results of experiments on plasma polymerization treatments of substrates intended for use within the body.
There has been a general lack of success and relatively few publications in the plasma polymerization biomaterials area (Yasuda, H. and Gazicki, M., Biomedical Applications of Plasma Polymerization in Plasma Treatment of Polymer Surfaces, Biomaterials vol. 3: 68, April 1982). It is believed that no glow discharge-coated implantable devices are available commercially, because the problems with the prior art are several. The difficulties with plasma polymerization techniques include variable plasma monomer forms in the gas, variable energies introduced to initiate polymerization, infinitely adjustable gas pressures and concentrations, and poor reactor designs. The prior art techniques generally produce non-uniform coatings or non-continuous coatings, especially on larger substrates. For example, some methods of coating tubes result in a heavy coating accumulation near the end of the tubing where the flow of gas is initiated. In treating the surfaces of elongate substrates there are typically regions along the length of the surface which are uncoated or which have a coating composition different from that intended.
Much of the prior art discloses substrate "treatment" as opposed to substrate "coating". "Treatment" involves extracting atoms and substituting others within the surface of the substrate, for example, a hydrogen atom in a polymer substrate is extracted and replaced with a fluorine atom. For example, U.S. Pat. No. 4,264,750 discloses a process whereby the surface of a hydrocarbon polymer may be fluorinated by replacing hydrogen atoms with a fluorinated species. "Treatment" techniques are generally limited to gases that will not polymerize, but rather form species which replace atoms in the substrate surface. The coating process, on the other hand, involves the build-up of a covalently bonded coating onto a substrate. This technique results in a three-dimensional cross-linked network upon the existing surface of the substrate.
It is an object of this invention to provide cross-linked polymerized coatings by glow discharge on the surfaces of elongate, e.g., tubular, substrates, and to control the ion density and field strength along the length of the tubes during plasma polymerization to provide a uniform coating. Another object is to coat both the inside and outside of a porous tube, so that the fluorine to carbon (F/C) atomic ratio along the length of the tube is substantially uniform, at least on its inside surface, and the F/C ratio of the coating on the interior surface of the tube is greater than the F/C ratio on the exterior. It is another object of the invention to provide a method for producing an F/C ratio gradient in a fluorine-containing coating through a porous substrate wall. Another object is to provide a coating comprising a substantially uniform three-dimensional, cross-linked fluoropolymer, covalently bonded onto a surface, and to provide prosthetic devices or other devices intended for implantation having a low surface energy. Another object of the invention is to provide a method of uniformly and reproducibly coating materials intended for implantation with a fluorine containing, low-energy surface coating having a F/C ratio greater than 1.5. Another object is to provide a porous, coated elastic prosthetic device having a coating which does not break when the device is stretched. Yet another object is to provide a prosthetic device with a thin film coating to achieve a substantially non-thrombogenic blood contact surface. Another object is to provide a process for uniformly coating implantable devices which does not alter or significantly change any mechanical or physical properties of the substrate, except those related to surface energy and composition.
These and other objects of the invention will be apparent from the description and claims which follow.